An auxiliary power unit (APU) system is often provided on an aircraft and is operable to provide auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a brushless, synchronous generator is coupled to and driven by the gas turbine engine during operation in a starting mode whereupon the generator develops electrical power.
As is known, an electromagnetic machine may be operated as a motor to convert electrical power into motive power. Thus, in those applications where a source of motive power is required for engine starting, such as in an APU system, it is possible to dispense with the need for the dedicated starter motor and operate the generator as a motor during the starting sequence to accelerate the engine to self-sustaining speed. This capability is particularly advantageous in aircraft applications where size and weight must be held to a minimum.
The use of a generator in starting and generating modes in an aircraft application has been realized in a variable-speed, constant-frequency (VSCF) power generating system. In such a system a brushless, three-phase synchronous generator operates in the generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads.
The generator of such a VSCF system is operated as a motor in the starting mode to convert electrical power supplied by an external AC power source into motive power which is provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion field windings via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
The use of single-phase AC excitation during operation in the starting mode can create problems due to the low power transfer capability across the exciter air gap. In order to provide sufficient main generator field current, a high AC voltage may be applied to the exciter field winding; however, application of such high AC voltage may create potential corona problems.
In order to improve the operation of a generator in the starting mode, the exciter portion of the generator may be modified, such as in U.S. Pat. No. 4,093,869 to Hoffman, et al.; however, modification of the exciter portion has disadvantages, and the need to modify the exciter portion precludes applicability of that concept to preexisting generators having standard exciter portions.
Lafuze, U.S. Pat. No. 3,902,073 and Stacey, U.S. Pat. No. 5,140,245 disclose starting systems for electromagnetic machines. Other systems for operating a brushless generator in a starting mode of operation are disclosed in Dhyanchand, U.S. Pat. No. 4,939,441, Dhyanchand, U.S. Pat. No. 5,013,929 and Glennon, et al., U.S. Pat. No. 5,068,590, all assigned to the assignee of the instant application.